1. Field of the Invention
The invention relates to a novel single- or two-stage process for preparing alkyl glycosides having C.sub.8 to C.sub.20 alkyl radicals and having a mean degree of glycosidation between 1.05 and 1.4.
2. Discussion of the Background
Alkyl glycosides having C.sub.8 to C.sub.20 alkyl groups can be prepared in whole or in part from renewable raw materials. Therefore, and also because of their very good biodegradability, they are becoming increasingly important. The products, in addition to their surfactant properties of interest, have the advantage that their polarity can be set exactly via the length of the alkyl chain and the degree of glycosidation. Because of this the alkyl glycosides can be specifically directed to their field of application.
In EP-A-0 495 174, a direct glycosidation of saccharides using C.sub.12 to C.sub.20 alcohols is described. In this single stage preparation of alkyl polyglycosides, sodium hypophosphite is added for color enhancement. Products are obtained here having mean degrees of glycosidation of 1 to 8. However, the preparation process is restricted to starting mixtures having a very low water content. It cannot, therefore, be carried out using an inexpensive syrup.
In the two-stage alkyl glycoside preparation, a glycosidation is first carried out, alkyl glycosides having short-chain alkyl groups being prepared from saccharides and C.sub.2 to C.sub.6 alcohols. These products are then converted into the desired alkyl glycosides having surfactant properties in the second stage by transglycosidation using C.sub.8 to C.sub.20 alcohols.
This preparation route has long been known. Recent applications in this sector frequently relate to the preparation of products improved in color. In these cases reducing agents are sometimes added or special equipment is sometimes used.
Thus in EP-A-0 501 032, the glycosidation is performed in an evaporator. The glucose syrup preferably used there is generally a starch hydrolysate containing oligosaccharides. The dextrose equivalent of such syrups is usually 90 to 95. The alkyl polyglycosides having long-chain alkyl groups prepared here preferably have mean degrees of glycosidation in the range from 1.3 to 5, a value of 1.65 being specified in the example.
According to EP-A-0 514 628, the transglycosidation can also be performed in an evaporator. In EP-A-0 514 627, the transglycosidation is carried out in a reaction column. A molar ratio of alkyl glycoside having a short alkyl radical to long-chain alcohol of 1:2 to 1:15 is set in this case and alkyl polyglycosides having degrees of glycosidation of preferably 1.2 to 3 are obtained.
WO 93/101 33 describes a two-stage process for preparing alkyl oligoglycosides, which can be carried out continuously or discontinuously and in which a glucose syrup having a monomeric glucose content of 90 to 100% is used. The process described here is highly complex overall. It requires a secondary reaction both in the glycosidation and also in the transglycosidation. Moreover, in both of the reaction stages narrow temperature ranges must be maintained. WO 93/101 33 furnishes alkyl oligoglucosides having a polyglucose content of less than 5 per cent by weight.
To separate off by distillation the fatty alcohols from alkyl polyglycosides having C.sub.10 to C.sub.18 alkyl groups and a mean degree of glycosidation of 1.05 to 1.4, according to DE-A-41 29 587, a thin-layer evaporator is used. The preparation of the alkyl polyglycosides is not considered in more detail there.
The publications mentioned do not give any indication as to how alkyl glycosides having a mean degree of glycosidation between 1.05 and 1.4 can be prepared in a simple manner.
The object of the present invention was therefore to provide a simple and inexpensive process for preparing alkyl glycosides having C.sub.8 to C.sub.20 alkyl radicals and having a mean degree of glycosidation between 1.05 and 1.4. The products, furthermore, are also to show a good color quality.